so would your suggestion be that in classical Latin, mediam diem appeared as part of phrases like ad mediam diem, and only in later Latin became lexicalized and conflated in meaning to meridiem?
– ColinJan 23 at 15:46

@Colin I can't really speak about later Latin, but meridies is certainly classical, and supposedly is conflated from merus (pure) rather than medius/-a and dies.
– Tom CottonJan 23 at 16:54

1

I think meridies is a dissimilation of medidies (at least that's what wiktionary says).
– ColinJan 23 at 18:38